1. Field of the Invention
The present invention relates generally to the compressed air and gas industry. More particularly, the present invention relates to the use of an apparatus and process for the removal of water from gas streams. Most particularly, this invention relates to an optimized compressed air system and method of operating the same utilizing a membrane air drier for systems which can operate at variable gas pressures with minimal loss of function or efficiency.
2. Discussion of the Related Art
Compressed gas systems generally consist of the following components: a power source, compressor, heat exchanger, particulate filter, aerosol coalescer, air-drier, accumulator, pressure regulator(s), check valves, and the equipment that the gas, such as air, is powering. In many compressed air systems, the compressed air consumption is less than the capacity of the compressor. There are several methods known in the industry to deal with these xe2x80x9cexcessxe2x80x9d compressed air situations.
One procedure is to run the compressor and store the compressed air in an accumulator at high pressure, and then to shut off the motor. A check valve can be placed at the inlet to the accumulator to prevent the stored compressed air from discharging back through the compressor. The motor remains off until the air pressure in the accumulator drops to a pre-set level, at which point the motor restarts and the compressor then refills the accumulator. Although this procedure may reduce energy usage, it is known in the art that frequent start-stop cycles can cause the motor to overheat, and may eventually cause damage to the compressor, motor, or the starting equipment.
An alternate method of control also uses an accumulator to store the compressed air. However, instead of shutting the motor off, a device can be placed in the system that reduces the pressure that the compressor is operating at, and discharges the excess air to atmosphere. One such device is an electrically operated unload solenoid. The pressure in the compressed air system prior to the accumulator is then regulated by the pressure drop across the unload solenoid. This type of control system is often known by the term xe2x80x9cconstant-runxe2x80x9d. Such xe2x80x9cconstant-runxe2x80x9d systems are popular, and are especially useful with smaller air compressors. However, such type of systems are prone to problems while the unload solenoid opens the system to atmosphere, as very little water is being removed from the system while the unload solenoid is open, and this can lead to very humid air being reintroduced into the accumulator when the unload solenoid closes.
In an attempt to avoid these problems, systems were developed using compressed air driers between the coalescer and the unloading valve. Such compressed air driers are devices that remove water from a compressed air system, specifically the water that exists in the vapor phase. These devices are generally placed prior to an accumulator to minimize the corrosive effects of moist air on the accumulator, although they can be placed after the accumulator as well. Several types of compressed air driers are known in the art, including refrigeration, desiccant, and membrane driers. Membrane air driers suitable for compressed air systems are known in the art. Several examples of membrane air driers can be found in U.S. Pat. No. 4,783,201 to Rice; U.S. Pat. No. 5,002,590 to Friesen; and U.S. Pat. No. 5,067,971 to Bixson.
Typically, air coming out of a coalescer is at, or near, ambient temperature, and is saturated with water vapor, any droplets of water having been removed by the filter and coalescer. The intent of using the membrane air drier is to remove as much of the water vapor as possible from the air before the compressed air reaches the accumulator. The performance of such air driers, which typically contain a membrane module, is measured in terms of the dewpoint suppression achieved as air passes through the module. The dewpoint is the temperature at which moisture will start to condense out of the moist air. Dewpoint suppression is how many degrees the dew point is lowered as the air passes through the drier.
The dewpoint suppression is a function of the membrane area, feed flow rate, operating pressure and temperature, and sweep fraction. The membrane air driers known in the art generally function by contacting one side of a semi-permeable membrane with a pressurized wet feed stream which is coming from the coalescer. The membrane is chosen such that it preferentially allows water vapor to permeate faster than the feed gas that is being dehydrated. A portion of the dried gas, as will be explained further herein below, and known in the art as the xe2x80x9csweepxe2x80x9d, is depressurized and fed back to contact the other side of the membrane, and acts to drive away the water moisture that has permeated the membrane.
The sweep of many membrane air driers is controlled by the use of an externally mounted valve or an internal orifice, and is usually factory preset for a given application. Membrane modules with externally mounted valves can be manually adjusted in service. However, there exists no means for cost effective automatic adjustment of the sweep based on the module feed conditions. Thus, for most purposes modules with externally mounted valves can be considered as fixed sweep modules, just like the internal orifice modules.
With fixed sweep membrane modules, the amount of sweep air is proportional to the operating pressure. Generally for fixed sweep modules operating before the accumulator in compressed gas systems utilizing an unload solenoid, the dewpoint suppression of the membrane module decreases when the unload solenoid opens, and the compressed gas flows through the membrane module at a lower pressure. This situation leads to higher humidity in the compressed air system between the membrane module inlet and the unload solenoid, and can lead to condensation when the unload solenoid closes and the entire system again increases in pressure. Some in the art have tried to prevent this condensation by appropriately sizing the fixed orifice membrane module for the low pressure condition when the unload solenoid is open. However, a significant disadvantage of this solution is that at normal operating pressures, the membrane module would be over designed and the sweep would be a much larger fraction of the feed gas than is necessary, leading to high operating costs. Thus, those in the field of compressed gas drying continue to try to optimize the design of membrane driers when they are used in gas circuits where the compressor duty is controlled by an unload solenoid placed downstream of the compressor.
The present invention provides a novel method for unloading compressed gas systems, as well as a novel membrane air drier which works under varying pressure without loss of drying efficiency, and the attendant problems of condensation attendant thereto.
In one embodiment of the present invention, a compressed gas system is provided with an accumulator having an inlet and an outlet; an unload solenoid having an inlet, a first outlet in communication with said inlet when said unload solenoid is open and a second outlet in fluid communication with said inlet when said unload solenoid is closed; and a membrane air dryer having an inlet and an outlet, the outlet of said membrane air dryer in fluid communication with said inlet of said unload solenoid; said membrane air dryer having a sweep inlet and a sweep outlet, said outlet of said unload solenoid which is in fluid communication with said inlet of said unload solenoid when said unload solenoid is open being in fluid communication with the sweep air inlet.
In a further embodiment of the invention, a compressed air system is provided which has seriatim, downstream of the compressor, all in fluid communication, an after cooler, a filter, a coalescer, a membrane air drier, an unload solenoid, and a receiver/accumulator. The unload solenoid, when opened, is in fluid communication with the sweep inlet of the membrane air drier.
In another embodiment of the present invention, a membrane air drier is provided having an air inlet, an air outlet, a sweep inlet, and a sweep outlet.
Thus, one of the objects of the present invention is to provide an improved method of operation for a compressed gas system.
Another object of the present invention is to provide an improved compressed gas or air system having a membrane air drier which has minimal loss of efficiency when operated at varying pressures.
A still further object of the present invention is to have a membrane air drier in fluid communication with a receiver/accumulator through an unload solenoid, and to have the unload solenoid unload directly into the sweep inlet of the membrane air drier.
Further objects and advantages of the present invention will be apparent from the following description and appended claims, reference being made to the accompanying drawings forming a part of the specification, wherein like reference characters designate corresponding parts in the several views.